Sounds more like hardware error than anything else so I'd suggest
disconnecting the HD and then seeing if the Ubuntu CD's will boot.

If not then try re-seating memory, and if you have more than one
module to try them singly. Basically, hardware fault-finding is a
process of re-seating / exchanging components until you find the
fault. Also worth bearing in mind that SATA and IDE cables can go bad
so worth swapping those too if nothing else works.

FWIW the update was probably coincidental but we'll know more after
the results of the above hardware checks.